The present invention relates to a homogenizer/subsampler for recovering representative analysis-sized subsamples from a bulk sample which has been taken from a plant process stream. The apparatus finds particular application in connection with the oil-water-solids mixtures encountered in hot water extraction process plants used to recover bitumen from tar sands.
Tar sand typically contains about 12% bitumen, 85% sand and fine solids, and 3% water. Manufacture of synthetic crude from this material only becomes economical at high throughput, since one requires 1 tonne of tar sand to produce 3/4 of a barrel of synthetic crude. The hot water extraction process used in this connection has been thoroughly described in the patent literature. A suitable description is given in Canadian Pat. No. 1,055,868, issued to K. C. Porteous et al. As can be seen from such descriptions, tar sand is mixed with hot water and thereafter, by a series of steps, the contained sand and clay solid matter is removed and disposed of in a tailings area and the bitumen is collected, first as a froth and then as substantially pure dry bitumen ready for upgrading.
At different points in the hot water extraction process, streams containing bitumen, water, and solids flow in very large volumes. Those who operate the process require analytical data to ensure that the process is working according to design conditions, that the bitumen reports to the froth, and that only minimal quantities of bitumen are lost to the various tailings streams. Unlike some chemical processes where conditions can be set at a desired optimum, the hot water extraction process must be continually adjusted due to the changing nature of the tar sand feed. It is particularly important that the process be adjusted as the levels of bitumen and fine clay materials in the feedstock vary.
Analytical procedures developed for tar sand streams give results of bitumen, water and solids content of the streams, particle size distribution of the contained solid matter, and characteristics of the bitumen. Such procedures are known to be accurate from the good agreement that can be obtained with identical samples. From the point of view of controlling the process however, accurate analysis alone is not sufficient. It is first necessary that the small sample used for analysis should be representative of the total stream from which it is taken. In many industries this is not a serious problem, either because streams are homogeneous or, where not, because simple subsampling techniques can be used to collect a representative analysis sample. Hence grab sampling, multiple grab sampling, riffling, and continuous withdrawal of a small side stream are well established techniques for reducing a large flow stream volume to an analysis-size volume.
There are difficulties associated with hot water extraction process streams when one comes to trying to obtain representative subsamples for analysis. More particularly, the heavier solid particles rapidly settle out of the mixture; bitumen, being sticky, adheres to any equipment it touches; and the fine clay either remains in contact with the bitumen or remains suspended in the water phase. These problems are complicated when diluent oils, such as naphtha, are included in the streams, largely because of the emulsion-forming tendencies of such diluents. The process streams in the plant vary in composition. Some may be high in bitumen, such as those near the product end of the process, others are high in water, such as the secondary tailings, and still others are high in solids, such as the primary tailings.
There has been a need, therefore, for a means for subsampling hot water extraction process streams in such a way as to provide representative analysis-size subsamples.
The present invention incorporates a structure arrangement known in the prior art. This known structure, used as a homogenizer, involves providing a draft tube coaxially mounted within a vessel. The draft tube is spaced from the vessel walls, so that an elliptical flow path is formed through the bore of the draft tube and the passage between the draft tube and the surrounding vessel. An impellor is provided in the draft tube to drive the vessel contents along the elliptical flow path and homogenize the components of the mixture forming such contents. Typical examples of such prior art homogenizers are shown in U.S. Pat. Nos. 1,351,352 (Stevens), 2,038,221 (Kagi), 3,189,080 (Overcashier), and 4,132,666 (Chikatsu).